GLASS 667 



it and the stone all round an interval of 3 or 4 inches, of which we shall presently see 

 the use. 



A cast plate, unless formed on a table quite new, has always one of its faces, the 

 one next the table, rougher than the other; and with this facing the roughing-down 

 begins. With this view, the smoother face is cemented on the stone table with Paris- 

 plaster. But often instead of one plate, several are cemented alongside of each other, 

 those of the same thickness being carefully selected. They then take one or more 

 crude plates, of about one-third or one-fourth the surface of the plate fixed to the 

 table, and fix it on them with liquid gypsum to the large base of a quadrangular 

 truncated pyramid of stone, of a weight proportioned to its extent, or about a pound 

 to the square inch. This pyramidal muller, if small sized, bears at each of its angles 

 of the upper face a peg or ball, which the grinders lay hold of in working it; but 

 when of greater dimension, there is adapted to it horizontally a wheel of slight con- 

 struction, 8 or 10 feet in diameter, whose circumference is made of wood rounded so 

 as to be seized with the hand. The upper plate is now rubbed over the lower ones, 

 with moistened sand applied between. 



This operation is, however, performed by machinery. The under plate being fixed 

 or imbedded in stucco, on a solid table, the upper one likewise imbedded by the same 

 cement in a cast-iron frame, has a motion of circumrotation given to it, closely resem- 

 bling that communicated by the human hand and arm, moist sand being supplied 

 between them. While an excentric mechanism imparts this double rotatory movement 

 to the upper plate round its own centre, and of that centre round a point in the lower 

 plate, this plate placed on a moveable platform changes its position by a slow hori- 

 zontal motion, both in the direction of its length and its breadth. By this ingenious 

 contrivance, which pervades the whole of the grinding and polishing machinery, a 

 remarkable regularity of friction and truth of surface is produced. When the plates 

 are sufficiently worked on one face, they are reversed in the frames, and worked 

 together on the other. The Paris-plaster is usually coloured red, in order to show 

 any defects in the glass. 



The smoothing of the plates is effected on the same principles by the use of moist 

 emery washed to successive degrees of fineness, for the successive stages of the opera- 

 tion ; and the polishing process is performed by rubbers of hat-felt and a thin paste 

 of colcothar and water. The colcothar, called also crocus, is red oxide of iron prepared 

 by the ignition of copperas, with grinding and elutriation. 



The last part, or the polishing process, is performed by hand. This is managed by 

 females, who slide one plate over another, while a little moistened putty of tin finely 

 levigated is thrown between. 



Large mirror-plates are now the indispensable ornaments of every large and sump- 

 tuous apartment ; they diffuse lustre and gaiety around them by reflecting the rays of 

 light in a thousand lines, and by multiplying indefinitely the images of objects placed 

 between opposite parallel planes. 



The process of silvering plate-glass will be described under MIEKORS. For the 

 production of the perfectly plane surface required to ensure true reflexion, a process 

 of the most careful polishing is required. It will not differ from the process already 

 described in any respect, beyond that of the extreme care which is required. 



Not only is this great exactness of surface required for the production of mirrors of 

 high class, but it is even more necessary in the plates of glass employed in the pro- 

 duction of ' the Ghost,' as it is called, which a few years ago excited so much of the 

 public attention at the Polytechnic Institution and elsewhere ; this phenomenon 

 being due to the total reflexion of highly-illuminated objects, from the plane sur- 

 face of highly-polished glass, the objects themselves being hidden from the spec- 

 tators. 



The progress of the plate-glass manufacture is very instructive. Owing to the 

 obstructions of Excise regulations, notwithstanding the reduction of the duty in 1819 

 to 60s. per cwt., the production in 1836 was but about 7,000 feet per week. In the 

 latter year, however, the manufacture ceased to be confined to two houses ; new 

 establishments were formed, and in 1845 the production had more than tripled, at a 

 great reduction of price. In 1845 the Excise duties on glass were abolished, and Sir 

 R. Peel, in proposing their abolition, said, If you leave the manufacture altogether 

 disburdened, as in France and Belgium, then, with your peculiar advantages of 

 material, the command of alkali and coal, my belief is, you will supply almost the 

 whole world.' And, thanks to the wisdom of that measure, freedom from fiscal 

 regulation has opened the door to extraordinary advantages, by means of which the 

 cost of production has been reduced to a point below that of any other country. In 

 cpaljthe savings effected by unrestricted action are no less than 80 per cent, in quan- 

 tity alone, involving a corresponding decrease in the cost of furnaces, fire-clay, pots, 

 tools, labour, and also in the erection of buildings that now suffice. The cost of coal, 



